1. Field of Invention
This invention relates to a system for balancing the load in a laundry appliance, particularly but not solely, a system for balancing the load in a horizontal axis washing machine.
2. Description of the Prior Art
Conventional horizontal axis washing machines involve a final spin cycle to extract the washed articles of as much of water as possible to reducing drying tie. However, the requirement of a high spin speed is at odds with quiet operation At the beginning of a spin the cycle the wash load can be quite severely unbalanced, such that when the machine tries to accelerate noise and stressful vibrations result.
The means that washing machine designers have employed so far to cater for imbalance in the load, is typically to suspend the internal assembly on springs and dampers in order to isolate its vibration, The difficulty is these suspension assemblies never isolate the vibration completely, and as the machine ages they deteriorate and the problem gets worse. Also, these suspension assemblies require significant internal clearance, and so valuable load capacity is lost when designing a machine to standard outside dimensions. Further, because the internal assembly must still withstand the forces due to the imbalance, considerable extra costs result.
The ideal approach is to eliminate the problem at its source, for which there are various solutions. The first possibility is to ensure that the wash load is evenly distributed prior to spinning. This is an effective solution but it is extremely difficult to achieve in practice. Therefore while steps can be taken to reduce the degree of imbalance that must be catered for, it is not possible to eliminate it sufficiently to ignore it there after. Another approach is to determine the size and nature of the imbalance, and add an imbalance that exactly counteracts the first.
Methods of compensating for imbalance in horizontal axis washing machines have been disclosed in U.S. Pat. No. 5,280,660 (Pellerin et al.), European Patent 856604 (Fagor, S. Coop). These disclosures relate to the use of three axially orientated chambers running the length of the drum, displaced evenly around the periphery of the drum, which when individually filled with water in the appropriate amounts can be used to approximately correct imbalances in the axis of rotation.
The disadvantage to these systems is that the imbalance may not be centered along the axis of rotation, and since no control is available along the axis of rotation this form of balancing will only ever be partially successful. This may mean that a suspension system may still be required to isolate the vibrations, which adds cost and may reduce the useful life of the appliance.
Static Imbalance
When an object of some shape or form is spun about a particular axis, there are two types of imbalance that it may exhibit: Static and Dynamic. Static imbalance is where axis of rotation does not pass through the Centre of Gravity (CoG) of the object. This means that a force, F, must be applied to the object (acting through the CoG) to keep accelerating the object towards the axis of rotation. This force must come from the surrounding structure and of course its direction rotates with the object, as illustrated in FIG. 1. There are two pieces of information required to define a static imbalance 3. They are the magnitude of the imbalance 1 (the moment of the CoG about the spin axis, which in SI units has dimensions kg m), and some angle 2 between the direction of the offset of the CoG and some reference direction within the object 4.
When mounted on a horizontal rotation axis, and under the influence of gravity, an object with a static imbalance will rotate until its CoG lies vertically under its axis of rotation. This also has the consequence that a horizontal axis machine, running at speeds slower than its resonance on its suspension and at constant power input, will exhibit a slight fluctuation in rotation speed as the CoG goes up one side and down the other. Unfortunately this is not a feasible technique for determining static imbalance at anything other than very slow speeds.
Dynamic Imbalance
Dynamic Imbalance is a little more complicated. In FIG. 2 the axis of rotation 5 is not parallel with one of the principle axes 6 of the object. The principal axes of an object are the axes about which the object will naturally spin.
For example, consider a short length of uniform cylinder 7 set to spin about its axis of extrusion, and thus is both statically and dynamically balanced Two weights are now attached to the inside of the cylinder, one 8 at one end and the other 9 at the other end but on the opposite side from the first one. The CoG 10 of the object has not been moved and so it is still statically balanced, but now spinning the cylinder will cause vibration; it has a dynamic imbalance. Static imbalance can be detected statically by seeing which way up the object rolls over to rest. Dynamic imbalance can only be detected with the cylinder spinning, i.e. dynamically.
It is an object of the present invention to provide a balancing system for a laundry appliance which goes as far as is practical for its purpose towards overcoming the above mentioned disadvantages.
Accordingly in a first aspect, the present invention consists in a laundry appliance comprising:
a perforated rotatable drum for dehydrating a clothes load,
a substantially rigid, free standing drum support means supporting said drum rotatably but non-translatably in relation to a support surface,
driving means for rotating said drum at speed thereby dehydrating the load, and
a system for compensating for imbalances of said drum and any load carried therein during dehydration of the load.
In a second aspect, the present invention consists in a laundry appliance having a perforated drum for dehydrating a clothes load, driving means adapted to rotate said drum at speed thereby dehydrating the load and a system for compensating for imbalances of said drum and any load carried therein during dehydration of the load, said system comprising:
first sensing means located at more than one position on the drum spin axis for detecting dynamic rotational imbalance in the load,
a digital processor which in use receives as inputs signals from said sensing means, and programmed to calculate the value and position of one or more masses required to be added to the drum to correct the sensed imbalance,
correction means for adding two or more masses to said drum, wherein in use at least one of said masses being axially spaced from the remainder of said masses and said processor controlling such additions such that the resultant value and position is substantially similar as the calculated value and position to correct the imbalance.
In a third aspect, the present invention consists in a laundry appliance having a perforated drum for dehydrating a clothes load, driving means adapted to rotate said drum at speed thereby dehydrating the load and a system for compensating for imbalances of said drum and any load carried therein during dehydration of the load, said system comprising:
first sensing means located at more than one position on the spin axis of said drum for detecting rotational imbalance in the load,
correction means for adding two or more masses to said drum to correct for any imbalance caused by the rotation thereof, and
a digital processor which in use receives as inputs signals from said sensing means and programmed with software causing said processor to carry out the following steps.
a) energising said driving means to apply a first predetermined rate of rotation to said drum;
b) instructing said correction means to add at least one small imbalance to at least one end of said drum and storing the detected rotational imbalances at each end of said drum;
c) determining the differential relationship between said at least one added imbalances"" and said detected rotational imbalances"" at each end of said drum, thereby estimating the value and position of one or more masses required to be added to the drum to correct the actual imbalance; and
d) controlling additions of one or more masses to said drum by said correction means such that the resultant value and position of the added masses is substantially similar to the said estimated value and position to correct the imbalance.
In a fourth aspect, the present invention consists in a laundry appliance having a perforated drum for dehydrating a clothes load, driving means adapted to rotate said drum at speed thereby dehydrating the load and a system for compensating for imbalances of said drum and any load carried therein during dehydration of the load, said system comprising:
first sensing means located at one or more positions on the spin axis of said drum for detecting rotational imbalance in the load with respect to the spin axis of said drum,
second sensing means located at one or more positions on the spin axis of said drum for determining the absolute acceleration of the spin axis of said drum,
a digital processor which in use receives as inputs signals from said first and second sensing means and programmed to estimate the value and position of one or more masses required to be added to the drum to correct the sensed imbalance,
correction means for adding one or more masses to said drum, said processor in use controlling such additions such that the resultant value and position of the added masses is substantially similar as the said estimated value and position to correct the imbalance.
The invention consists in the foregoing and also envisages constructions of which the following gives examples,